The present invention relates to a device for triggering the destruction of a selected portion of a hydraulic structure, and to a hydraulic structure including such a devices
The invention applies in particular to an earthfill or rockfill embankment dam, dike, or levee, or to a composite dam or dike built in part as an embankment and in part out of concrete or masonry. The dike may be a front dike (across a water course) or a side dike (along a water course, to protect surrounding land from flooding). If the structure is a dam, it may be any kind of embankment or composite dam that creates a reservoir of water, or a saddle dam associated with the above-specified dam.
In numerous hydraulic structures of the above-indicated kind, it is known to provide privileged break points which, under exceptional flooding that is in danger of destroying the structure, give way at predetermined locations of the structure which are selected so as to minimize damage caused to the structure itself and/or to people or property flooded by the structure breaking. One of the major problems associated with such a system is that of satisfying the following criteria:
it must be very stable and reliable during normal operation of the hydraulic structure; and
it must be very unstable when exceptional events occur that threaten the survival of the hydraulic structure.
The system that is presently in most widespread use for this function is known as a "fuseplug dike". It comprises either a portion of the hydraulic structure itself or else a dike built at a distance from the hydraulic structure at some other point on the periphery of the reservoir, e.g. in a saddle. The top of the fuse-plug dike is positioned or levelled at a level such that water spills over it only during exceptional flooding. This level is higher than the normal operating level (RN) of the hydraulic structure in which the fuseplug dike is integrated or with which it is associated, but it is lower than the maximum water level (RM) that the structure is designed to withstand. The fuseplug dike is built of materials suitable for ensuring that it is destroyed by hydraulic erosion in the event of water spilling over its top. The principle on which such a fuseplug dike operates is simple, but it suffers from uncertainty concerning the water level at which the erosion phenomenon will begin and as to the speed with which said fuseplug will be destroyed.
The resistance to erosion of an embankment dam or dike depends on numerous parameters, and in particular:
the thickness of the nappe passing over the top of the dike; PA1 the duration of the spillage; PA1 the nature of the materials constituting the embankment of the dike and the density thereof (and thus in particular the extent to which the embankment is compacted in an earthfill dam); PA1 the standard cross-section of the dike; PA1 the gradient of the downstream slope which, together with the thickness of the nappe, determines the speed water flows over the downstream slope; and PA1 the presence or absence of protection on the downstream slope; for example the presence of grass or other vegetation on the downstream slope increases the resistance of the dike to erosion.
Since fuseplug dikes must destruct only for flooding of very low probability, in particular exceptional floods of the kind that occur once in 100 years or in 1,000 or more years, there is uncertainty concerning how some of the above-mentioned parameters will vary over time (e.g. the plant cover on the downstream slope).
Tests on scale models and real-life experience on embankment dams or dikes that have suffered spillage over their tops have shown that they are sometimes capable of withstanding nappes that are several tens of centimeters thick (i.e. the height of the nappe is several tens of centimeters) for several hours (see the report of the 16th International Congress on Large Dams, Q.63-R.35, 13-17 Jun. 1988, pages 560 to 569, and in particular Table 1 on page 563). As a result, in the event of an exceptional flood, if the fuseplug dike is not destroyed rapidly, water can continue to accumulate upstream of the dike and can rise to a level that presents a danger for the remainder of the hydraulic structure before the fuse-plug dike has been destroyed.
In an attempt to remedy this drawback, proposals have been made to create at least one pilot channel or breach in the top of the fuseplug dike with the bottom thereof being at a level that is lower than the level of the top of the fuseplug dike, so that water passing over the bottom of the pilot breach also attacks the flanks thereof and thus destroys the fuseplug dike more quickly (see the report of International Symposium on Dams and Exceptional Floods, Grenada, 16 Sep. 1992, Volume III, article by Nelson L. de S. PINTO, pages 34 to 39, and article by Dr. Chonggang SHEN, pages 71 to 83, FIG. 1b). However, it has been observed that in the event of spillage, erosion does not begin at the bottom of the pilot breach but begins lower down, at the foot of the downstream slope of the dike, and the flanks of the pilot breach are eroded to a significant extent only after the portion of the downstream slope situated beneath the bottom of the pilot breach has itself been destroyed. From the point of accuracy concerning the water level at which destruction of said fuseplug begins and from the point of view of speed of such destruction, a fuseplug dike with a pilot breach therefore provides little improvement over a fuseplug dike without a pilot breach.
Proposals have also been made to place a cylinder having a diameter of 4 feet (about 1.2 m) at the top of the impervious core of the fuseplug dike, immediately beneath the bottom of the pilot breach, which cylinder is embedded in the embankment of the dike (see the report of the U.S. Committee on Large Dams, Modification of Dams to Allow Large Floods to Pass, 12th Series of USCOLD Annual Conferences, Forth Worth, Tex., April 1992, article by Paul F. Bluhm et al., pages 1 to 25, FIG. 7). In the event of exceptional flood, the water pouring through the pilot breach erodes the sand that is to be found in front of the cylinder. After a certain length of time, once the sand has been expelled in front of the cylinder, the cylinder itself is expelled by the water, thereby releasing a rush of water whose thickness corresponds to the diameter of the cylinder. The rush of water released in this way accelerates the erosion of the pilot breach. Although such a known system does indeed make it possible to obtain rapid erosion of the pilot breach once the cylinder has been expelled by the water, the time at which the cylinder is expelled and the level that had been reached at that time by the water upstream from the fuseplug dike cannot be determined accurately in advance. The time and the level depend, in particular, on the speed with which erosion takes place in front of the cylinder. The speed of such erosion itself depends on numerous parameters such as those mentioned above concerning erosion of the dike, and some of the parameters can change over time between the time at which the dike was built and the time at which it needs to be destroyed by an exceptional flood. In addition, it has been observed that because of its weight the cylinder sinks partially into the impervious core of the dike and the core tends to retain the cylinder prior to it being expelled by the water. Here again, with the known system, it therefore remains uncertain as to how quickly the fuseplug dike will be destroyed and as to the level which the water will have reached immediately before the dike is destroyed.
An object of the present invention is thus to provide a device enabling destruction of a selected portion of a hydraulic -structure such as an embankment dam, dike, or levee confining a water reservoir or a water course to be triggered reliably and quickly when the water level reaches a predefined level, and in particular enabling the destruction of a fuseplug dike or of any other selected portion of a structure that is built out of erodible materials so as to enable it to be destroyed by hydraulic erosion.